Injection molding is one of the well-known techniques for the preparation of sintered zirconia. See U.S. Ser. No. 07/588,180 filed Sep. 26, 1990 now abandoned or EP 0 420 284 A2 published Apr. 3, 1991. Sintered zirconia is generally prepared by blending zirconia powder with an organic binder to form a molding composition and injecting the composition into a mold to form a green body of predetermined shape, followed by binder removal and firing.
The mold which is used for the injection molding of ceramics including zirconia generally has a cavity defining a shape conforming to a desired product and a gating system including a sprue, a runner, and a gate to the cavity. The compound is melted in a cylinder of the injection molding machine. The molten compound is introduced to the sprue of the mold from a nozzle at the distal end of the cylinder and then fills the runner until it reaches the gate. Past the gate, the compound fills the cavity where it is cooled and solidifies. Finally, the mold is opened and the molded compound or green body is removed therefrom. Since the green body has a burr corresponding to the sprue and runner at this point, the burr must be cut off before the desired green body can be obtained.
In the prior art, molds for use in injection molding ceramic compounds are designed to have a sprue with a large draft or taper, a thick runner, and a gate with a large cross sectional area since the ceramic compounds are considerably lower in flow, ability to fill very narrow areas, and green strength than plastics.
In particular, the sprue and runner which do not form part of the final molding were necessary and indispensable in the prior art ceramics injection molding technique. From the standpoint of efficient use of a source compound, the sprue and runner (so called "cold runner") portions were recovered and recycled. This recycle procedure, however, had the drawbacks of increased contamination and deteriorated flow of the compound. Further, the process required extra steps of cutting the molding from the runner portion at the gate and machining the residual gate portion away from the molding.
Included in the plastics injection molding technique is a special molding technique using a mold free of sprue and runner which is known as a "sprue-runnerless molding or hot-runner molding." See Japanese Patent Application Kokai (JP-A) Nos. 30143/1985 and 206613/1985. Although the sprue-runnerless molding technique provides efficient use of a source compound, its application is limited to those plastic compounds featuring good flow. There is found no example in which the technique is successfully applied to ceramic and other compounds having increased filler loadings.
We previously found that a zirconia powder is suited for injection molding when it meets an optimum balance of particle shape, average particle size and specific surface area (U.S. Ser. No. 07/588,180 now abandoned or EP 0 420 284 A2). Continuing research works, we have found that the sprue-runnerless injection molding technique which has been limited to well flowing plastic compounds can be applied to a zirconia compound under specific conditions.